The present invention relates to the field of semiconductor devices. More specifically, the present invention relates to forming photoresist features on a semiconductor wafer.
The size of photoresist features vary with respect to photoresist thickness for a particular wafer substrate. Therefore, it is important to determine a photoresist thickness that will provide the desired photoresist feature size. Conventional processes for determining a thickness that will provide a desired photoresist feature size require the generation of a swing curve.
A swing curve is typically generated by preparing a dozen or more identical semiconductor wafers. The semiconductor wafers can be bare wafers or can have other layers and/or structures formed thereover. The layers and/or structures formed over the semiconductor wafer typically include those layers and/or structures that may affect photoresist feature size in the fabrication process. For example, when a photoresist layer is to be formed over a metal layer, the metal layer is deposited over each of the dozen or more semiconductor wafers.
The dozen or more semiconductor wafers are then placed in a photoresist coat track. Each wafer is then coated with the type of photoresist that is to be used in the semiconductor fabrication process. The track recipe for each wafer is different so as to coat each wafer with a different thickness of photoresist. The thickness of the photoresist is then measured on each of the dozen or more twelve semiconductor wafers.
All twelve semiconductor wafers are then exposed and developed so as to produce an identical photoresist feature on each of the dozen or more wafers. For each semiconductor wafer, the size of the photoresist feature is measured. The size of the photoresist feature is plotted relative to the photoreisist thickness. Typically, thickness of photoresist is plotted on the x-axis and size of resist feature is plotted on the y-axis. A curve (swing curve) is then generated that fits the plotted points.
The process of generating a swing curve is typically expensive and time consuming due to the number of test wafers that must be fabricated and measured. Also, random or systematic process variation between wafers can result in the generation of a swing curve that is not accurate. When the swing curve is not accurate, costly fabrication defects occur, resulting in reduced yield and potentially resulting in device failure.
Thus, there is a need for a method for generating an accurate swing curve. In addition, there is a need for a method for inexpensively and quickly generating a swing curve. Also, there is a need for a method for forming a photoresist feature having a desired size. The present invention meets the above needs.
The present invention provides a method for quickly and inexpensively generating an accurate swing curve. In addition, the present invention provides for forming a photoresist feature having a desired size.
A method for generating a swing curve is disclosed in which a layer of photoresist having varying thickness is formed over a semiconductor wafer. The thickness of the layer of photoresist is measured at a plurality of points on the semiconductor wafer. The semiconductor wafer is then exposed and developed to produce a photoresist structure that includes a plurality of features. For each of the points at which thickness was determined, the size of a corresponding feature is determined. This gives a set of photoresist thickness measurements and a set of corresponding size measurements. A curve (swing curve) is then determined that correlates size measurements and thickness measurements. The swing curve accurately indicates the size of features that can be expected for any given thickness of photoresist.
The swing curve of the present invention can be determined using a single semiconductor wafer. Thus, there is no need to prepare a dozen or more semiconductor wafers. Also, there is no need to deposit, expose, and develop photoresist and no need to measure features on each of the dozen or more semiconductor wafers as is required in prior art methods for forming a swing curve. This results in significant savings in time and cost. Also, by using a single semiconductor wafer, process variation that results from generation of numerous test wafers is avoided. This gives a swing curve that is more accurate than prior art methods for forming a swing curve.
In one embodiment, the swing curve is used to determine a desired optimum thickness for photoresist deposition. This thickness is then used in a semiconductor wafer fabrication process to form a photoresist layer having the desired thickness. The photoresist is then exposed and developed to produce photoresist features. Because the method of the present invention produces an accurate swing curve, the resulting features will have the desired size. Also, process latitude with respect to process variations is maximized. This minimizes manufacturing defects resulting from photoresist size variance, giving increased yield and reduced device failure.